1. Field of the Invention
The present invention relates to an organic electroluminescent device (hereinafter, abbreviated to an “organic EL device”) which is an electroluminescent device used in various displays such as the display for cellular phones and various light sources, and to a method for manufacturing the same, and more particularly to an organic EL device comprising a polymeric luminescent material in an organic thin film, which can be driven in a wide range of brightness, from that of low brightness used in various display applications to high brightness used in light sources.
2. Description of the Related Art
Generally, organic EL devices are light emitting devices which utilizes the phenomenon of electroluminescence exhibited by solid fluorescent substances, and such organic EL devices have been widely put to practical use as miniature displays.
Organic EL devices can be classified to two types by the difference in the materials used in the light emitting layer. One type of organic EL devices uses an organic compound of low molecular weight in the light emitting layer, which is mainly produced by means of vacuum vapor deposition. The other type is polymeric organic EL devices which utilize a polymeric compound in the light emitting layer, which the present invention is related to.
Polymeric organic EL devices enable thin film formation by means of spin coating, ink-jet method, printing or the like by using a solution dissolving the material constituting each functional layer, and thus have drawn interest as a technology in which low production costs or device enlargement can be expected with simple and convenient processes.
A typical polymeric organic EL device is prepared by laminating a plurality of functional layers such as a charge injection layer, a light emitting layer and the like between an anode and a cathode. An explanation on the constitution of such a polymeric organic EL device and its manufacturing method will be given below.
First, a thin film of PEDOT:PSS (a mixture of polythiophene and polystyrenesulfonic acid; hereinafter, referred to as PEDOT) is formed as a charge injection layer by means of spin coating or the like, on a glass substrate onto which an ITO thin film has been formed as an anode. PEDOT is a substance practically used as a standard material for a charge injection layer, and being disposed adjacent to the anode, it functions as a hole injection layer.
A film of polyphenylenevinylene (hereinafter, referred to as PPV) and its derivatives, or of polyfluorene and its derivatives is formed as a light emitting layer on the PEDOT layer by spin coating or the like, and onto this light emitting layer, a film of metal electrode is formed as a cathode by vacuum vapor deposition to complete the device.
As such, a polymeric organic EL device has an excellent feature that the device can be produced by a simple process and has seen a variety of applications. However, there are still two problems to be solved, such as that sufficiently great luminescence intensity cannot be obtained, and that the device does not have a sufficient life property when driven for a long period of time.
Reduction, or deterioration, of the luminescence intensity of a polymeric organic EL device proceeds proportionally to the electric current passing through the device multiplied by the time for current flow. However, details of the process are not yet known, and an extensive study thereon is still being carried out.
Although there are many suspected causes for lowering of the luminescence intensity, deterioration of PEDOT is considered as one dominant cause. PE DOT, as described above, is a mixture of two polymeric materials such as polystyrenesulfonic acid and polythiophene, among which the former polymer is ionic and the latter has localized polarity in the polymer chain. Such Coulomb interaction attributable to anisotropy of charges allows moderate bonding between the two polymers and thus the excellent charge injection property of the material.
In order for PEDOT to exhibit excellent properties, intimate interaction between the two polymers is essential. However, a mixture of polymeric materials in general is likely to undergo phase separation owing to the delicate difference in the solubility to a solvent, and this is not an exception to PEDOT (Applied Physics Letters, Vol. 79, pp. 1193-1195). To undergo phase separation means that the moderate bonding of the two polymers is relatively easily breakable, and it implies that when PEDOT is driven in an organic EL device, it may be possibly unstable, or as a result of phase separation, may have adverse effects on other functional layers upon diffusion of a component not involved in the bonding, in particular an ionic component, caused by the electric field resulting from electric current flow. Thus, despite its excellent charge injection property, PEDOT is not considered as a stable substance.
In regard to such problems associated with PEDOT, it has been proposed to abandon PEDOT (Applied Physics Letters, Vol.79, pp.1193-1195). In this Non-Patent Document 1, it is proposed to use a silicon dioxide (SiO2) layer having an electron-blocking action in place of a PEDOT layer. Even though this certainly improves the efficiency of the device as compared with the case where nothing is disposed between the ITO electrode and the light emitting layer, the properties of the device are rather poorer when compared with a device having a PEDOT layer.
It has been also proposed to insert a buffer layer having an electron-blocking function in a device with a PEDOT layer, between the PEDOT layer and the light emitting layer (Applied Physics Letters Vol. 80, pp. 2436-2438). When an electron-locking layer is inserted, there is an increase in the carrier density in the vicinity of the interface between the light emitting layer and the electron-blocking layer, thus improving the luminescence efficiency. Since an improvement of luminescence efficiency means an increase in the luminescence intensity with respect to the input power, in order to obtain an equivalent light intensity, the electric current passing through the device is decreased, and consequently deterioration of the PEDOT is lowered, thereby its durability being improved. However, as the current density is further increased, there reaches a realm where a further increase in the current density does no longer lead to an increase in the luminescence intensity. Therefore, there is a limit in the brightness obtained, and it is not possible to obtain higher brightness. Thus, it cannot be said to have the brightness obtainable at a sufficiently satisfactory level, and the durability is not sufficient, either.
Especially when such device is used in an exposure head as the light source, it requires the property of high brightness. Thus, extensive research is being conducted to meet the demand on further enhancement of the brightness.
Thus, polymeric organic EL devices have been illustrated. As described above, organic EL devices include a group of so-called polymeric organic EL devices using low molecular weight materials in the light emitting layer, and there are many proposals for improving the luminescence property for the group.
For example, in Patent Document 1 and Non-Patent Document 3, reduction in driving voltage of the device is attempted by laminating a thin film of oxides of vanadium (V), molybdenum (Mo), ruthenium (Ru) or the like, in place of the ITO electrode or on the ITO electrode. In these examples, the reason for insufficient durability of the device is considered to be attributable to the high barrier between the electrode and the hole transport layer or the light emitting layer and too much voltage applied on this barrier. Thus, reduction in the driving voltage and improvement in durability are attempted by using a thin film of metal oxide whose work function is greater than that of conventional anode material, ITO, and thereby lowering the barrier between the electrode and the hole transport layer or the light emitting layer (Japanese Patent Laid-Open Publication No. 93771 and Journal of Physics D: Applied Physics Vol. 29).